In diesel vehicles, particularly in heavy vehicles such as heavy trucks, the SCR method, i.e. a method for selective reduction of nitrogen oxides, is used to clean the vehicle's exhaust gases so that those released into the surroundings do not contain high contents of nitrogen oxides.
In the SCR method, a liquid urea solution injected into the exhaust pipe may be used as NOx reducing agent.
Using liquid urea solution may entail problems at low exhaust temperatures in that the reducing agent injected does not become vaporised quickly enough, with consequent formation in the exhaust system of deposits which may lead to pressure drop. This presents a problem which is solved by maintaining such a high exhaust temperature that the reducing agent injected becomes vaporised in the exhaust pipe.
EP 2 078 834 A refers to a method and a system for cleaning of exhaust gases at such a high exhaust temperature that an additive becomes gasified such that the exhaust gases are warmed to shorten the time required before injection of liquid additive can commence, which involves using a great deal of energy.
US 2005/0204733 A1 refers to a device in which a thermoelectric element is used to generate electrical energy.
In the SCR method, gaseous ammonia injected into the exhaust pipe may also be used as reducing agent. Using a gaseous reducing agent reduces problems due to deposits of reducing agent in the exhaust pipe, since they only occur if the exhaust temperature before the exhaust gases reach the catalyst falls so low that the reducing agent is converted from gas to liquid or solid.
DE 10 2007 058 768 A1 refers to a device and a method for post-treatment of exhaust gases whereby gaseous reducing agent, e.g. ammonia, is injected into the exhaust pipe, and energy present in the exhaust gases is used to vaporise the reducing agent before it is injected in the exhaust pipe.